Generation of tunable light pulses

ABSTRACT

A device for generating tunable light pulses has a non-linear optical fiber. The optical spectrum of femtosecond light pulses can be modified by this optical fiber, taking advantage of solitonic effects. In order to make available such a device, which makes it possible to vary the pulse energy and the wavelength of the light pulses independently of one another, an optical compressor precedes the non-linear optical fiber.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] Applicants claim priority under 35 U.S.C. §119 of GermanApplication Nos. 103 08 249.2 and ______ filed Feb. 25, 2003 and Feb.23, 2004, respectively.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a device for the generation oftunable light pulses, having a non-linear optical fiber, by means ofwhich the optical spectrum of femtosecond light pulses can be modified,taking advantage of solitonic effects.

[0004] 2. The Prior Art

[0005] Laser systems that are able to produce femtosecond light pulsesare increasingly being used in basic physical research and also in otherareas of research. Using such laser systems, it is possible to observerapid physical, chemical, and biological processes essentially in “realtime.” Commercial areas of use for laser systems that producefemtosecond light pulses exist in the fields of materials examinationand processing, in the field of medicine, as well as in the so-called“life science” field. Concrete applications that can be mentioned asexamples are multi-photon microscopy as well as optical coherencetomography.

[0006] Particularly in the field of single-photon and multi-photonmicroscopy, there is a need for laser systems that are bothhigh-performance and spectrally variable, which are inexpensive and easyto operate. Until now, femtosecond light pulses having a highperformance in the laboratory were usually generated by means oftitanium-sapphire laser systems. It is a disadvantage of these systemsthat they are very expensive, complicated to adjust, and difficult tohandle. Also, the through-tunability of the optical spectrum of thegenerated light pulses is not satisfactory in such laser systems.

[0007] Nowadays, there is a tendency towards generating femtosecondlight pulses with pulse energies of one nanojoule and more by means ofpurely fiber-based laser systems. Such systems usually consist of apulsed laser light source, which emits femtosecond light pulses in theenergy range of 100 picojoules. These light pulses are then amplified,by means of an optically pumped amplifier fiber, so that the lightpulses are available in the desired pulse energy range.

[0008] For example, a device for generating tunable light pulses ispreviously known from EP 1 118 904 A1. This device works with a specialnon-linear optical fiber. By means of this fiber, the optical spectrumof femtosecond light pulses that are provided by a suitable pulsed laserlight source can be modified, in targeted manner, taking advantage ofsolitonic effects. To vary the spectrum of the generated light pulses,the intensity of the light coupled into the non-linear optical fiber isvaried in the system described in the cited reference. This approachdirectly results in the disadvantage that in the previously knownsystem, the desired optical spectrum of the generated light pulses isdependent on the pulse energy. An independent variation of the pulseenergy and the wavelength of the light pulses is therefore not possible,using the previously known system. Another disadvantage is that in thepreviously known system, the non-linear fiber used must have a length ofseveral 10 m, so that the desired solitonic optical effects becomeactive to a sufficient degree. Because of the long running distance, anundesirable loss of coherence of the generated light pulses can occur.

SUMMARY OF THE INVENTION

[0009] Against this background, it is an object of the present inventionto provide a device for generating tunable light pulses, in which thedisadvantages known from the state of the art are avoided. Inparticular, it is an object to generate widely tunable light pulses,whereby the pulse energy and the wavelength of the light pulses can bevaried, independent of one another.

[0010] These and other objects are achieved, according to the invention,by providing a device for the generation of tunable light pulses havinga non-linear optical fiber in which the non-linear optical fiber ispreceded by an optical compressor.

[0011] As has been shown in the device according to the invention,non-linear processes in the fiber into which the light-pulses arecoupled result in the formation of two separate light pulses in thefiber. The spectrum of these pulses is shifted towards the long-wavespectrum and the short-wave spectrum, respectively, as compared withthat of the light pulse that was coupled in. In this connection, thespectral separation of the light pulses is adjustable by means of theoptical compressor, which precedes the non-linear optical fiber in thedevice according to the invention. The temporal frequency behavior or“chirp” of the light pulses that are coupled in is influenced, intargeted manner, by the optical compressor. The optical spectrum that ismodified by means of the non-linear optical fiber then dependssensitively on the predetermined “chirp,” so that the desired tunabilityof the light pulses exists. It is particularly advantageous that theoptical spectrum of the light pulses generated by means of the deviceaccording to the invention can be varied independent of the pulseenergy.

[0012] Experiments have shown that the device according to the inventionadvantageously makes do with a very short non-linear optical fiber,having a length of only a few centimeters, to achieve the desiredmodification of the optical spectrum of the light pulses. In this way,coherence losses of the generated light pulses are effectivelyprevented.

[0013] The light pulses coupled into the non-linear optical fiber of thedevice according to the invention should have a pulse energy of at leastone nanojoule. Such high pulse energies are desirable, so that thesolitonic optical effects occur to the required degree, in order togenerate the tunable light pulses within the non-linear optical fiber.

[0014] It is practical if the optical compressor of the device accordingto the invention is configured to be adjustable, so that the temporalfrequency progression of the light pulses coupled into the non-linearoptical fiber is changeable. This arrangement makes it possible, inconvenient and simple manner, to tune the generated light pulses to thedesired wavelengths, in that the adjustable elements of the opticalcompressor, such as prisms or optical lattices, are adjusted in suitablemanner.

[0015] According to an advantageous embodiment of the device accordingto the invention, the non-linear optical fiber is configured to maintainpolarization and shift dispersion. Such a fiber is described, forexample, in the article by T. Okuno et al. in the journal IEEE Journalof Selected Topics of Quantum Electronics, Volume 5, page 1385, 1999.The solitonic optical effects that are mentioned, which result in thedesired modification of the spectrum of the light pulses according tothe invention, occur in the non-linear optical fiber if the wavelengthof the light pulses coupled into the fiber lies in the range of the zerodispersion wavelength of the fiber. In experiments, a non-linear opticalfiber having a zero dispersion wavelength in the range of 1.52 μm wasused to generate tunable light pulses, using the device according to theinvention.

[0016] Light pulses having a particularly broad optical spectrum can begenerated, using the device according to the invention, if thenon-linear optical fiber has a particularly small diameter of ≦5 μm. Inexperiments, a fiber having a core diameter of 3.7 μm was successfullyused, whereby a fiber length of only 7 cm proved to be sufficient. Thisfiber results in a usable wavelength range for tuning the light pulses,using the device according to the invention, which extends from about1.1 μm to 2.0 μm.

[0017] In addition to conventional optical glass fibers, microstructuredphotonic fibers can also be used as non-linear fibers for generating thetunable light pulses, according to the invention. Such fibers have atransverse microstructure in the region of the core. By means ofsuitably adapting the zero dispersion wavelength, as well as by means oflow core diameters, and thereby a high level of non-linearity of suchcrystal fibers, it is possible to generate widely tunable light pulsesaccording to the invention.

[0018] Optionally, in the device according to the invention, thenon-linear optical fiber can be followed by an additional opticalcompressor, in order to achieve light pulses having a minimal pulseduration at the output of the device. In experiments, the use of a prismcompressor using prisms made of SF10 glass has proven itself. It waspossible to achieve pulse durations of ≦25 femtoseconds with thiscompressor.

[0019] In a further embodiment, an optical measurement element isprovided for characterizing the light pulses modified by means of thenon-linear optical fiber. Experiments have proven the use of aspectrometer as well as the known Frequency Resolved Optical Gating(FROG) structure.

BRIEF DESCRIPTION OF THE DRAWING

[0020] Other objects and features of the present invention will becomeapparent from the following detailed description considered inconnection with the accompanying drawing. It should be understood,however, that the drawing is designed for the purpose of illustrationonly and not as a definition of the limits of the invention.

[0021] In the drawing,

[0022] The single figure, FIGURE 1, schematically shows an embodiment ofthe device according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0023] In the structure schematically shown in FIGURE 1, a pulsed laserlight source 1 is provided, which emits femtosecond light pulses havinga pulse energy of more than one nanojoule. It is advantageous if thelaser light source is a completely fiber-based system that is composedof a commercially available pulsed fiber laser and an optical pumpedamplifier fiber that follows the laser. The use of conventionalfree-beam lasers as laser light source 1 is also possible, however. Thetemporal progression of the light pulses emitted by laser light source 1is predetermined, in targeted manner, by means of a prism compressor 2.In the exemplary embodiment shown, the light pulses run through theprism arrangement twice, for this purpose. The double arrow indicatesthat one of the prisms of the compressor is adjustable, in order tothereby be able to tune the generated light pulses, according to theinvention. Prism compressor 2 is followed by a non-linear,dispersion-shifted and polarization-maintaining optical fiber 3, intowhich the light is coupled by means of a lens 4. The light pulsescoupled into fiber 3 have a wavelength that essentially corresponds tothe zero dispersion wavelength of optical fiber 3. Because of thenon-linear solitonic effects that occur in fiber 3, the optical spectrumof the light pulses is severely modified. The light pulses that exitfrom optical fiber 3, which are coupled out by means of another lens 5,have an optical spectrum that is sensitively dependent on the “chirp”predetermined by means of compressor 2. By means of adjusting thecorresponding prism in compressor 2, the light pulses that exit fromfiber 3 can be adjusted in the wavelength range between 1.1 μm and 2.0μm. As described above, the optical spectrum of the light pulses at theoutput of fiber 3 has two separate components, which are shifted towardsthe long-wave and the short-wave spectrum range, respectively, ascompared with the wavelength of the light pulse that was coupled in. Anadjustable spectral separation of the two components by more than 100THz can be achieved using the structure shown. Even though a shortnon-linear optical fiber 3, which can have a length of ≦10 cm, issufficient, according to the invention, the light pulses run apart,dispersively, within fiber 3. This dispersion can be compensated bymeans of an additional prism compressor 6. When using SF10 glass prisms,tunable light pulses having a pulse duration of ≦25 femtoseconds wereimplemented, using the structure shown in the drawing. To characterizethe light pulses, a FROG structure or a spectrometer 7 is provided.

[0024] It should be pointed out that according to the invention, otherdispersive optical components besides prism compressor 2 can also beused for a targeted adjustment of the “chirps” of the light pulsescoupled into fiber 3, such as lattice compressors, so-called “chirped”mirrors, fiber Bregg lattices, additional dispersive optical pathsegments, etc., for example.

[0025] Although only at least one embodiment of the present inventionhave been shown and described, it is to be understood that many changesand modifications may be made thereunto without departing from thespirit and scope of the invention as defined in the appended claims.

1. A device for generating tunable light pulses comprising: (a) a pulselaser light source for producing femtosecond light pulses having anoptical spectrum; (b) a non-linear optical fiber for modifying theoptical spectrum of the femtosecond light pulses, said optical fibertaking advantage of solitonic effects; and (c) an optical compressorpreceding said non-linear optical fiber.
 2. The device according toclaim 1, wherein the light pulses coupled into said non-linear opticalfiber have a pulse energy of at least one nanojoule.
 3. The deviceaccording to claim 1, wherein said optical compressor is adjustable topermit changing the chirp of the light pulses coupled into saidnon-linear optical fiber.
 4. The device according to claim 1, whereinsaid non-linear optical fiber maintains polarization or isdispersion-shifted.
 5. The device according to claim 1, wherein saidnon-linear optical fiber has a core diameter of less than fivemicrometers.
 6. The device according to claim 1, wherein said non-linearoptical fiber comprises a microstructured photonic fiber.
 7. The deviceaccording to claim 1, wherein said non-linear optical fiber has a lengthof less than one meter.
 8. The device according to claim 1, furthercomprising an additional optical compressor following said non-linearoptical fiber.
 9. The device according to claim 1, further comprising anoptical measuring instrument for characterization of the light pulsesmodified by means of said non-linear optical fiber.